Most actions in MnM are governed by the player rolling against DC (Difficulty Class). Resisting the effects of being damaged in no different.
When a character gets hit the +# that the attacker is add to 15 to create a DC.
The character hit rolls a d20 and adds the appropriate Save +# (usually Damage) and is the rolled number is equal or great the damage is avoided. If the rolled # is less than the DC you calculate the damage result.